Abstract: Poly(3-hexylthiophene) (P3HT) has been used in CsPbI₃ perovskite solar cells (PSCs) as the hole transport layer (HTL), due to its excellent hydrophobic property and cheap cost. However, the inevitable defects in perovskite lead to the limited photovoltaic performance of CsPbI₃ PSCs. In this study, diethylammonium iodide (DEAI) was doped as an additive to passivate the defects in P3HT based CsPbI₃ perovskite. Considering the diethylamine cations can effectively form hydrogen bonds with halide ion, the DEAI doping method could not only reduce the defect density but also facilitate the extraction and transport of carriers in the device. The optimal power conversion efficiency of the device prepared with DEAI doping method increases from 14.68% to 16.75%. In addition, the stability of the device is also significantly upgraded due to the improvement of CsPbI₃ perovskite film. This work can provide reliable theoretical and experimental evidence for further PSCs research.